"Tim Shoppa" wrote in message roups.com...
Are "overtone" crystals cut differently than "fundamental" crystals?
Or are they just specified differently?

In particular, say I took a garden-variety 20MHz fundamental
microprocessor crystal and instead used it at its fifth overtone,
trying to hit 100 MHz. The LC network is there to make sure that it's
on its fifth overtone. Will this "misuse" mean that the oscillator
will be harder to start up, less stable, more noisy, ???, than a
crystal oscillator made out of a real overtone crystal? I don't mind
if I "miss" 100 MHz by a several tens or hundreds of ppm, as long as
it's stable there.

If anyone knows of a place that ships off-the-shelf 100 MHz fifth or
seventh overtone crystals, I can avoid this whole exercise.... :-)

Tim.


Tim,

to get optimum performance one would grind the 100MHz 5.OT finer or
even polish it, and the thickness of the electrodes might be different
to get optimum Q.
But you should be ok by using a 20MHz fundamental in its 5th.

There are also manufacturers that make 100 in fundamental (up to about
200MHz), and many should have 100 in 5th as standard part...

Frank

In message , Frank Moe
writes
There are also manufacturers that make 100 in fundamental (up to about
200MHz), and many should have 100 in 5th as standard part...
High frequency fundamentals are real and can be purchased to at least
250MHz note they are expensive, their Q which is ultimately material
related reduces with increased frequency (loss is per cycle) such that
it may be no higher than a SAW resonator . Still better than a SAW as
the temperature coefficient for the SAW only has the linear cancelled
whereas the AT cut has the parabolic term cancelled.
Finally the 5th overtone will only pull 1/25 times the fundamental.
Finally finally the SAW can be run at much higher power levels so noise
floor is much better.
Finally finally finally a SAW needs a mask which gives a high up front
cost and lead time.
The difference in frequency for overtone /X fund may be much more than
2000ppm where the plate is not very parallel and the plate back and
electrode diameter non optimum. Beware AT cut strip crystals ie a long
section of a circular diameter these may not like overtone operation.



not pull with external reactance change perhaps a byt
--
dd